Method of electrostatic printing of multiple copies



NOV. 21, 1967 KENMl T U T NI ET AL 3,354,464

LE COPIES METHOD OF ELECTROSTATIC PRINTlNG OF MULTIP Filed Jan. 17, 1964 (n-IH ml IIIEHIIIIIYQTQIII [SENSE M1 m'izi mizk United States Patent W 3,354,464 METHOD OF ELECTROSTATIC PRHQTING 0F MULTIPLE CQPIES Kenmi Tsukatani, Tokyo, Masaaki Nagamine, Kawasaki, Yuji Hamada, Yamagata-shi, and Kiyoshi Takagi, Kawasaki, Japan, assignors to Fujitsu Limited, Kawasaki, Japan, a corporation of Japan Filed Jan. 17, 1964, Ser. No. 338,345 Claims priority, applicafzion Japan, Jan. 21, 1963, 8 863 Claims. 61. 346--74) FIG. 3 shows schematically and in section a portion of v a different sheet material prepared and used in accordance with the invention;

FIG. 4 illustrates schematically an electrostatic printing apparatus according to the invention;

FIG. 4a shows on a larger scale a sheaf of sheets being simultaneously imprinted in the apparatus;

FIG. 5 is a planar development of type faces embodied in the apparatus of FIG. 4; and

FIG. 6 shows schematically the arrangement of selector electrodes also forming part of the apparatus shown in FIG. 4.

It is an object of our invention to provide a method and means for simultaneously producing an electrostatic impression in the form of a latent image upon more than two sheets of paper within a single sheaf, with satisfactory results as regards the sheets located inside the sheaf. The problems underlying this object, and the solution afforded by the invention will be explained presently.

According to the known principle of electrostatic printing or recording, the sheet material that is to receive the imprint consists of dielectric material having an extremely high insulation resistance (specific resistance) such as, for example, polystyrene. The sheet may consist entirely of such material, or the high-resistance material may constitute a thin coating on a base of electrically conductive material. For producing a print, the recording sheet is placed between two electrodes of which one has a front face constituting the type or other pattern to be reproduced. A high voltage is then impressed between the two electrodes to produce an electric field through the recording sheet. While this is being done, the sheet may be at standstill, or it may be advanced relative to the two electrodes, or vice versa. The electric field causes an electric charge to occur at the highly resistive surface of the sheet on an area corresponding to the configuration of the pattern electrode. Thus an electrostatic latent image of the pattern is produced with an electric polarity and charge density depending upon the applied voltage. Thereafter the sheet is removed from the electrodes and subjected to a developing process which converts the latent electrostatic image to a visible and permanent image. This is done by contacting the sheet surface with powdered dielectric coloring material that electrostatically adheres to the charge areas and thereafter is fixed thereto, for example by a heat treatment if the powder contains thermoplastic constituents. .Various methods based upon the above-described principle are known, the applicable developing process for converting the latent image to a permanent printed image being substantially in accordance with those employed in xerography.

3,354,464 Patented Nov. 21, 1967 As a rule, a recording or reproducing method of this kind serves to produce only one copy at a time, but fails if an attempt is made to pile several sheets upon each other for simultaneously impressing them electrostatically with latent images. This will be understood from the fact that, when a plurality of sheets are placed upon each other and simultaneously subjected to the electrostatic field between the electrodes, a latent image formed by an electrostatic charge will appear on both sides of each sheet (with the exception of the one adjacent to the backing electrode). The two charged images on each sheet have mutually opposed electric polarities and therefore tend to interfere with each other so as to cause partial or substantially complete neutralization. It has been found that when a sheaf of recording sheets, thus simultaneously subjected to an electrostatic printing field, is subsequently divided into individual sheets, the developing process fails to produce clear images due to the above-mentioned electric charge troubles and discharge phenomena.

Tests of the just-mentioned type have been performed in the manner apparent from FIG. 1, which will first he described only to the extent necessary for illustrating the tests, but will be more fully referred to in a later place in conjunction with a description of the invention proper.

Shown in FIG. 1 is a pattern electrode 1 and a counter or backing electrode 2. A sheaf of three recording sheets 3 is located between the electrodes. The contact surface of the pattern electrode 1 forms a type face indicating a letter, numeral, code or other pattern to be reproduced on the recording sheets. Each recording sheet was 1 inch wide and 0.5 mil thick, consisting of polyethylene terephthalate. The three individual sheets 31, 3-2, 3-3 were jointly displaceable between the electrodes for repeating the electrostatic operation, but were kept stationary with respect to the electrodes during the interval in which a signal voltage was applied betweeen the pattern electrode 1 and the counter electrode 2. A voltage of 1500 v. was supplied from a source 7 of direct voltage under control by a control device schematically shown at 8. Whenever the voltage was applied, the resulting electric field between the electrodes was supposed to form an electrostatic latent image on each of the sheets. After a number of individual operations, a strip of electrostatic images was to appear along the sheet material in parallel relation thereto. However, the subsequent development of the sheets showed throughout an unsatisfactory appearance of the imprints and thus revealed the occurrence of electric charging and discharging trouble and also the occurrence of peeling greatly impairing and destroying the latent images.

In theory, such failure could be remedied by exposing the back surface of the individual sheets, as they emerge from the electrodes and from the sheaf, to a corona discharge for the purpose of neutralizing the disturbing surface charge by ionization. This method, however, is difficult to apply, and it is not easy to control the proper amount of ionization, particularly if the recording sheets are kept traveling at relatively high speed. This remedy therefore is far beyond practical applicability.

As mentioned above, it is an object of our invention to eliminate charging, discharging and latent image production defects and to obtain a clear electrostatic latent image on each sheet piled in a sheaf or bunch and jointly subjected simultaneously to a single electrostatic operation at a time. It is also an object of our invention to achieve such an improvement to secure a substantially uniform quality of the respective images on the individual sheets, without the necessity of performing the latent-image production at standstill or reducing the desired travel speed of the record sheets, nor the speed of response at which the latent images are electrostatically recorded.

Still another object of our invention, in conjunction with those mentioned above, is to permit using electrostatic record sheets which, with respect to appearance and use, are similar to ordinary paper.

To achieve these objects, and in accordance with features of our invention, we simultaneously place between electrodes of the type described with reference to FIG. 1 a sheaf of recording sheets. Each individual sheet comprises a paper base which is coated at least on one side with a layer of dielectric recording material. The dielectric recording material has a specific resistance higher than ohm-cm. and a finely distributed surface irregularity or roughness whose roughness depth is between 2 and microns. By applying a high voltage between the electrodes across the thickness of the entire sheaf, a latent electrostatic charge image is formed simultaneously on the dielectric coating of each sheet. The sheets are thereafter separated prior to subjecting them to the above-mentioned developing process to convert the latent image into a visible and permanent print.

In conjunction with the above-mentioned features, it is preferable for best results to provide the back surface of the paper base sheet with a relatively high electric conductance, namely a specific resistance of 10 to 10 ohm-cm.

The last-mentioned features relating to back surface conductivity are known as such in conjunction with dielectric films of polystyrene or other plastic material and are discussed first.

If the back surface of the thin dielectric recording sheet is coated with material of relatively good electric conductivity and placed between the electrodes, across which a voltage is impressed, an external electrical field or electrostatic latent image cannot form the back surface of the conducting coating but appears only on the opposite surface. If several such sheets, each having the back surface coated with electrically conducting material of 10 to 10 ohm-cm. specific resistance, are sandwiched between the electrodes, the applied voltage causes an electric field to extend through the entire sheaf, and after the electrodes are removed, the external electric field due to the electrostatic latent images does not appear on the back surfaces but occurs only on the opposite, dielectric surfaces. When such a sheaf is thereafter separated into individual sheets, even at high speed, no trouble is encountered and a clear image is obtained on each sheet. Apparently, in the presence of the electric field, the backing layer of 10 to 10 ohm-cm. resistance acts somewhat as a dielectric material, but also approaches the effect of a metal backing with respect to the electrostatic condition. However, the thickness of the backing layer of electrical conductivity is limited by the definition or clarity desired of the electrostatic latent image in each sheet. That is, it has been found that in practice the conducting backing layer is limited in thickness to a maximum of about 0.05 mm.

The following methods are applicable for producing such a backing layer on sheet material of high insulation resistance:

(1) Sufficient electric conductivity can be provided on one surface by absorption of water by the polar group in a molecule of a surface-activating agent coated on one side of the highly resistant sheet.

(2) A surface layer of electric conductivity is provided by water absorption of the polar group generated by such chemical treatment as saponification, performed on one side of the highly resistant sheet consisting of synthetic resin.

(3) Electric conductivity is provided by coating an organic semiconductor solution upon the sheet to form a film thereon.

(4) Electric surface conductivity is provided by depositing in vacuum a semiconductor substance, such as selenium, or a semiconductor compound upon the highly insulating sheet.

(5) A film of high insulation resistance is formed on sheet material previously treated with an antistatic reagent.

By using recording sheets prepared as described above or produced in an equivalent manner with a surface of improved electric conductance, multiple sheets can be simultaneously provided with latent electrostatic images. However, with most plastic sheet materials, even if processed in the manner described so far, the manufacture, storage or handling of film-type materials leaves much to be desired. In cases where the printed sheet is to be handled in ofiices, libraries or wherever else it is necessary to incorporate the sheets in folders and envelopes like other papers, it is especially preferable to provide a type of recording sheet which, to all appearances and with respect to handling, is essentially similar to ordinary paper.

For the foregoing reason, in accordance with our invention the base layer in the individual recording sheets is made of ordinary paper which is coated on one side with polystyrene or other suitable dielectric material having the above-mentioned surface roughness of fine distribution in comparison with the images to be recorded. The back of the paper is preferably rendered more conductive, for example by one of the above-mentioned methods, so that it has a specific resistance of 10 to 10 ohm-cm. When such paper base sheets are utilized, all the individual sheets within a sheaf can be reliably provided with uniform and satisfactory latent images by a single electrostatic operation of improved simplicity and stability of performance. This will be explained with further reference to FIG. 1, and in conjunction with FIGS. 2 and 3.

Each of the individual sheets Zl, 32 and 33 in the sheaf between the pattern electrode 1 and counter electrode 2 of FIG. 1 consists essentially of a base layer b and a recording layer a as schematically shown in FIG. 2. The recording layer a consists of highly insulating material having a specific resistance above 10 ohm-cm, such as polystyrene or the other plastic or plastics mentioned hereinafter. The base layer b consists of paper. After a voltage is applied between the two electrodes, a latent image consisting of an electrostatic charge is produced on each of the three sheets. Thereafter the sheaf is separated, and the individual sheets are developed to produce a visible image in accordance with the above-mentioned known electrophotographic developing process. To avoid the trouble encountered in past attempts at multiple-sheet =printing of this kind, the recording layer a of high insulating resistance is provided with fine surface irregularities which are uniformly distributed over the entire surface area and have a roughness depth between 2 and 20 microns. The plastic surface coatings with the surface irregularities face the pattern electrode 1.

When the sheaf comprises sheets of the present invention, of the aforeclescribed type, a clear print is obtained on each individual sheet after developing the latent image.

To provide the desired irregularities on the surface of the plastic coating a (FIG. 2), the paper used for the base layer [1 may also have the same irregularities from the outset so that these irregularities appear on the coated surface after the plastic coating is applied to one side-of the paper. However, the paper used for the sheets may also have a smooth surface obtained by finishing processes such as sizing, filling or calendering, and only the coated thin film of plastic or other high-resistance material may be given the desired surface roughness. It has been found that it is not essential whether or not the back surface of the paper is smooth or uneven, this being of no effect upon the clearness of the electrostatic latent image. It has further been found by experimentation that the finer the distribution of the surface irregularities on the recording side, the better will be the definition of the image, and that the practically applicable depth of the irregularities is in the abovementioned range between 2 and 20 microns.

In fact, when a record is produced on only one copy, the quality of definition and clearness of the electrostatic latent image increases with an increase in smoothness at the surface of the recording layer a, so that from this viewpoint the use of a smooth paper base b with a smooth plastic coating a as shown in FIG. 3 would appear to be preferable. This, however, does not apply to the simultaneous production of two or more copies. When for this purpose a smooth recording surface according to FIG. 3 is used, a clear recording is obtained only on the first sheet 3-1 adjacent to the pattern electrode, whereas there is no clear formation of any image on the other sheets 3-2 and 33.

A variation in specific resistance of the paper used as base material, as may result from changes in air humidity, does not impair the characteristics of the sheet with respect to the electrical charges, even if the rec-ording-sheets are left exposed to air having a relative humidity of approximately 20% to 75%. In the event that prolonged exposure to atmospheric humidity in excess of this range is to be expected, it is advisable to further improve the sheets by forming an electrically conductive layer of film, not on the back surface of the paper, but between the base paper 12 and the recording layer a to thus provide an intermediate zone, as shown at c in FIG. 2, whose specific resistance is limited to a value of 'to' 10 ohms. This intermediate layer can be produced for example by any of the above-described methods.

The upper limit of thickness for the paper base in practice is about 50 microns. The definition and clearness of the resulting image decreases when the thickness is increased beyond this limit. The charge density of the electrostatic latent image depends upon the intensity of the electric field applied to each recording medium.

When the image is observed under a magnifying glass or microscope, it is found that the electrostatic force of the latent image causes powdered coloring material to adhere mainly in the indented portions of the surface irregularities, whereas no adherence of the powder is observed on the protruding portions or ridges. Hence the electric charge seems to substantially occur in the indentations of the slight and finely distributed surface roughness. This has the further advantage that the latent image is not damaged by conveying or handling of the paper, even if the paper is contacted by guide or transporting rollers. This facilitates the construction of practical equipment needed for automatic processing of this kind.

The paper base I) of the sheet may be coated with a thin film of high-resistance material on only one side as described, or such a thin coating may be provided on both sides of the paper, at least one coated side then possessing the required unevenness or roughness at the surface that is to receive the image. In either case, the back surface is rendered more conductive by one of the methods described in the foregoing. Aside from the thin coating being of polystyrene, it may also comprise other synthetic plastics known as suitable for such dielectric purposes, for example polyethylene terephthalate, polytetrafiuorethylene or the like.

A preferred embodiment of apparatus made and used according to the invention will be described with reference to FIGS. 4 to 6.

According to FIG. 4, the apparatus comprises a rotary cylindrical electrode 11 which carries on its peripheral surface the code letters or other characters 12 to be transferred to the recording sheets. The code characters 12, arranged as shown in FIG. 5, are formed by raised portions in the same manner as ordinary printing type faces, and the intermediate indented portions of the electrode surface are filled with material of high electric insulation resistance, such as polystyrol, so that the conducting type faces are flush with the cylindrical outer surface of the electrode. The cylindrical electrode with its conductive type faces forms part of an electric circuit of the type shown in FIG. 1. According to FIG. 4, the rotary elec trode 11 and consequently all character units 12 are grounded.

The rotary electrode cooperates with a number of stationarily mounted selector electrodes 13 of electrically conductive material arranged as shown in FIG. 6. Denoted by 14 in FIG. 4 is a sheaf of recording sheets which are separately shown at 14-1, 14-2 and 14-3 in FIG. 4a. Schematically indicated at 15 in FIG. 4 is a high-voltage driver circuit and at 16 a control stage.

When the apparatus is in operative condition, the typeface electrode 11 rotates at constant speed in the direction indicated by the arrow, and the selector electrodes 13 register within the respective typeface characters on the rotary electrode 11 respectively, being radially spaced therefrom an amount sufficient to conveniently permit the insertion of a sheaf of recording sheets.

After the sheaf 14 is inserted between the rotary typeface electrode 11 and the bank of selector electrodes 13, the selector electrodes are pressed toward the rotary electrode 11 with some amount of pressure for maintaining a good contact with the sheaf 14. During operation the sheaf of sheets is advanced between the electrodes in the downward direction as indicated by another arrow.

When the required type-face 12 on the rotary electrode 11 reaches a position facing a selected electrode 13, the control stage 16 actuates the high-voltage circuit 15 to apply a high voltage between the fixed selector electrode and the type-face electrode, for example 1500 v. or more depending upon the number of sheets in the sheaf. As a result, a latent image consisting of an electrostatic charge is formed on the slightly rough surface of each recording sheet 14-1, 14-2, 14-3 facing toward the type-face electrode 11. The operation is analogously repeated for different type-face characters and different lines of the recording sheets. The advancing sheaf is separated into individual sheets on which a visible image is fixed by electrophotographic or xerographic development, causing powder material to adhere electrostatically to the latent image and to be subsequently fixed thereto such as by thermal treatment of thermoplastic coloring powder.

As mentioned, the electrostatic voltage to be applied between the two electrodes depends upon the number of sheets that form the sheaf, aside from the fact that the optimum voltage also varies with different sheet compositions and thicknesses. Recording sheets used with the above-described apparatus according to FIGS. 4 to 6 were made of base paper 40 microns thick of the type employed for tracing paper and correspondingly having a dull and relatively rough finish. A film of polystyrene was coated on only one side of this paper to produce the recording layer of the desired degree of slight surface irregularity. The opposite, uncoated side of the paper was left in its original surface condition and consequently exhibited the same degree of surface roughness. With three of these sheets combined in a sheaf and having the plastic recording layers facing the side of the rotary electrode 11, the voltage applied between the electrodes was approximately 1800 v. Under these conditions a clear and stable latent electrostatic image was obtained on each sheet, and no difliculty was encountered in developing it into a satisfactory visible image by the conventional xerographic developing process, such as the cascade process, the electromagnetic brush process, an openchamber process, or by means of liquid development as also employed in ordinary electrophotography.

By comparison, when a calendered smooth tracing paper of 40 microns thickness was used as a base and was coated on one side with the same polystyrene material and by the same method, it was found infeasible to produce clear images on all of the three sheets of a sheaf. A latent electrostatic image was found to appear only on the plastic film contacting the rotary type-face electrode 11, but no latent electrostatic image was formed on the other films.

A voltage of about 2000 volts between the two electrodes was found satisfactory for simultaneously producing latent images on four sheets made of a base paper 38 microns thick, of the type used for carbon tissue paper, and coated with a toluene solution of polystyrene, so as to exhibit at the coated surface the fine irregularities of the paper. The same irregularities caused by the fibers in the paper were permitted to remain on the opposite side. In this case, too, any of the conventional developing methods was found applicable for obtaining a visible clear image on each of the four sheets.

The invention can also be used in cases where it is desired to print not only copies on paper but also upon a transparent sheet to be subsequently available for reproduction purposes such as for blueprinting. For this purpose, at least one transparent recording sheet is incorporated into a sheaf otherwise consisting of paper base sheets of the present invention as described above. Such a transparent sheet may consist of the above-mentioned dielectric synthetics known for electrostatic printing purposes, for example polyethylene terephthalate, polytetrafluorethylene or polystyrene. When thus including a transparent sheet in the sheaf, it is preferable to have at least one recording sheet with an ordinary paper base located at the side of the type-face electrode. It has been found that in this manner a clear electrostatic charge image is formed on each sheet of the sheaf including the transparent material.

We claim:

1. The method of electric printing by electrostatically producing a latent image on insulating sheet material between backing electrode means and printing-face electrode means and thereafter developing a visible means by applying dielectric coloring substance to the image, which comprises simultaneously placing a number of sheets as a sheaf between the two electrodes, each individual sheet comprising a paper base coated on one side with a layer of dielectric recording material of more than 10 ohmcm. specific resistance, said dielectric recording material having finely distributed surface irregularities of a roughness depth between 2 and 20 microns, applying voltage between the electrodes across the thickness of the sheaf whereby a latent electrostatic charge image is formed simultaneously on the coating of each sheet, and then separating the sheaf into individual sheets prior to developing the visible irnages thereon.

2. The electrostatic printing method according to claim 1, wherein the paper base forms a layer of increased electric conductivity corresponding to a specific resistance of 10 to 10 ohm-cm, said layer being situated on the coated side of the paper.

3. The electrostatic printing method according to claim 1, wherein said sheets consist of a rough-surfaced paper base having on at least one side a synthetic plastic coating to serve as said recording material, said coating exhibiting substantially the surface roughness of said paper base with said roughness depth between 2 and 20 microns.

4. The electrostatic printing method according to claim 1, wherein the opposite side of the paper base forms a surface layer of increased electric conductivity corresponding to a specific resistance of 10 to 10 ohm-cm.

5. The method of electric printing by electrostatically producing a latent image on insulating sheet material between backing electrode means and printing-face electrode means and thereafter developing a visible image by applying dielectric coloring substance to the image, which cornprises simultaneously placing a number of sheets in the form of a sheaf between the two electrodes, each individual sheet being formed of a paper base coated on one side with a layer of dielectric recording material of more than 10 ohm-cm. specific resistance and having finely distributed surface roughness of a depth between 2 and 20 microns, displacing the sheaf relative to the electrode means and simultaneously applying voltage between said respective electrode means across the thickness of the intermediate area of the sheaf so as to successively form latent images on a larger total area of the coating on each sheet, and then separating the sheaf into individual sheets prior to developing the visible images thereon.

References Cited UNITED STATES PATENTS 3,210,185 10/1965 Olden.

BERNARD KONICK, Primary Examiner. L. J. SCHROEDER, Assistant Examiner. 

1. THE METHOD OF ELECTRIC PRINTING BY ELECTROSTATICALLY PRODUCING A LATENT IMAGE ON INSULATING SHEET MATERIAL BETWEEN BACKING ELECTRODE MEANS AND PRINTING-FACE ELECTRODE MEANS AND THEREAFTER DEVELOPING A VISIBLE MEANS BY APPLYING DIELECTRIC COLORING SUBSTANCE TO THE IMAGE, WHICH COMPRISES SIMULTANEOUSLY PLACING A NUMBR OF SHEETS AS A SHEAF BETWEEN THE TWO ELECTRODES, EACH INDIVIDUAL SHEET COMPRISING A PAPER BASE COATED ON ONE SIDE WITH A LAYER OF DIELECTRIC RECORDING MATERIAL OF MORE THAN 10**13 OHMCM. SPECIFIC RESISTANCE, SAID DIELECTRIC RECORDING MATERIAL HAVING FINELY DISTRIBUTED SURFACE IRREGULARITIES OF A ROUGHNESS DEPTH BETWEEN 2 AND 20 MICRONS, APPLYING VOLTAGE BETWEEN THE ELECTRODES ACROSS THE THICKNESS OF THE SHEAF WHEREBY A LATENT ELECTROSTATIC CHARGE IMAGE IS FORMED SIMULTANEOUSLY ON THE COATING OF EACH SHEET, AND THEN SEPARATING THE SHEAF INTO INDIVIDUAL SHEETS PRIOR TO DEVELOPING THE VISIBLE IMAGES THEREON. 